We have the 340 hp Cummins 6.7 liter engine and have been VERY happy with it. The 2000 series transmission may not have the capacity of the 3000 one, but we have had no trouble towing a 2800 lb. car over 48000 miles. And in addition it is the smoothest Allison transmission I have ever had. An Allison tech told me that it is not just because it is newer, but they actually are smoother shifting.
Regarding power, what I have learned is that this engine holds it max torque to over 2700 rpm, so it is governed at that rpm instead of the typical 2200-2400 rpm. This means that I can climb a hill at speeds up to 60 mph in 4th gear. Because of this I have found that I can climb mountains as well or better than our last motorhome with the 9 liter, 400 hp CAT engine in it.
In summary, we are VERY pleased with that engine and transmission.